1. Field of the Invention
The present invention relates generally to the broad area of sensing devices and more specifically to a sensing device that includes a plurality of layers of electret material as sensing elements.
2. Description of the Prior Art
One common definition of an electret is a dielectric material permanently polarized by heating the material and placing it in a strong electric field during cooling. Electrets are commonly classified on the basis of pyroelectric or piezoelectric properties, both of which produce an electrical signal due to a change in dipoled moment. Although pyroelectric and piezoelectric materials are often thought of as two distinct classes of electrets, actually piezoelectric materials are a generic class in which pyroelectric materials are included as a species. Thus, a large number of materials have been pyroelectric and piezoelectric characteristics.
Use of electrets having both pyroelectric and piezoelectric properties has been made in forming various types of sensing devices as evidenced by U.S. Pat. No. 3,769,096 to Ashkin et al., which discloses a device for pyroelectric sensing of incident radiation, and U.S. Pat. No. 3,768,059 to Ayers et al., which teaches a strain sensor employing piezoelectric principles. The type of electrets described in the above patents have both pyroelectric and piezoelectric properties. Thus, the Ashkin et al. sensor is susceptible to producing an erroneous sensing operation as a result of spurious piezoelectrically produced electrical signals and similarly, the sensor in the Ayers et al patent has the same susceptibility because of pyroelectrically produced signals.